A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, one or more parameters of the patterned substrate are typically measured, for example the overlay error between successive layers formed in or on the substrate. There are various techniques for making measurements of the microscopic structures formed in a lithographic process, including the use of a scanning electron microscope and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and one or more properties of the scattered or reflected beam are measured. By comparing one or more properties of the beam before and after it has been reflected or scattered by the substrate, one or more properties of the substrate may be determined. This may be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with a known substrate property. Two main types of scatterometer are known. A spectroscopic scatterometer directs a broadband radiation beam onto the substrate and measures the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. An angularly resolved scatterometer uses a monochromatic radiation beam and measures the intensity of the scattered radiation as a function of angle. An ellipsometer also measures polarization state.
In order that the radiation that impinges on the substrate is diffracted, an object with a specific profile and pitch is printed on to the substrate and is often known as a scatterometry object or scatterometry profile. The object may be a diffraction grating or the like, which is made up of an array of bars or other periodic structures. The cross-section of the structures, as seen from the surface of the substrate upwards, is known as the profile. Ideally, the object (or a plurality of different objects) that is printed on to the substrate would have a predetermined shape and would be printed perfectly each time it was printed. However, because of the small size (ranging from, for example, 32 to 250 nm) of the object, its size is very sensitive to processing variations of all types. Accordingly, it is desirable to have a system to determine how exactly the object is shaped, i.e., know the profile of the object.
Generally, the way in which the profile of a scatterometry object may be determined is by diffracting a beam of radiation from the object and comparing the diffraction pattern with model diffraction patterns that are stored in a library of diffraction patterns alongside the model profiles that create these model patterns. For example, United States patent application publication US 2003/0028358 describes a system in which an actual signal from a scatterometry object is compared with a library of stored signals and the system tries to find the closest match of signals. The stored signals are each linked to an object profile parameter. An object profile parameter may be, for instance, the critical dimension (CD), a width of the object (which may vary with height), the height of the object or the angle of a side surface of the object (this angle being measured either from the surface of the substrate or from a normal to the substrate surface). It then goes on to describe the method of finding a closest match of a signal with each parameter of the scatterometry object. In other words, various possible parameters and possible permutations of parameters are tested to find a combination that gives rise to a signal that is as close as possible to the actual signal that has come from the scatterometry object. This gives a series of iterations of a “model signal”. This method is repeated iteratively until the model signal is as close as possible to the actual signal and then the model signal is stored alongside the parameters used. Finally, a computer checks the database comprising the parameters to determine if all parameter combinations have been entered.